This invention relates to high solids solvent based coating compositions having a low VOC (volatile organic content) and in particular to a clear coating composition useful for refinishing clear coat/color coat finishes of a vehicle such as an automobile or a truck.
Clear coat/color coat finishes for automobiles and trucks have been used in recent years and are very popular. Kurauchi et al U.S. Pat. No. 4,728,543 issued Mar. 1, 1988 and Benefiel et al U.S. Pat. No. 3,639,347 issued Feb. 1, 1972 show the application of a clear coat to a color coat or basecoat in a xe2x80x9cwet on wetxe2x80x9d application, i.e., the clear coat is applied before the color coat is completely cured.
There is a need for a clear coating composition that can be used to repair these clear coat/color coat finishes that has a low VOC to meet with pollution regulations for refinishing vehicles, that cures rapidly to a tack free coating at ambient temperatures, and that can be buffed in a relatively short period of time thereafter to a high gloss finish. Such a combination of properties is not provided by the prior art coatings, as for example, as shown in Lamb et al U.S. Pat. No. 5,286,782 issued Feb. 15, 1994 and Barsotti et al U.S. Pat. No. 5,763,528 issued Jun. 9, 1998.
The novel coating composition of this invention has the aforementioned desirable characteristics.
A coating composition containing about 40-90% by weight of film forming binder and 10-60% by weight of an organic liquid carrier;
wherein the binder contains about
(a) 10-70% by weight, based on the weight of the binder, of a dispersed gelled acrylic polymer having
(i) a core of gelled polymerized ethylenically unsaturated monomers which is not soluble in the organic liquid carrier and having chemically grafted thereto
(ii) substantially linear stabilizer polymeric components that are soluble in the organic liquid carrier and comprise polymerized ethylenically unsaturated monomers and have a weight average molecular weight of about 500-20,000 determined by GPC (gel permeation chromatography) using polystyrene as the standard;
wherein the core, the stabilizer polymeric component, or both contain at least 3% by weight of polymerized ethylenically unsaturated monomers having isocyanate groups attached thereto that are capable of reacting with component (b);
(b) 30-90% by weight, based on the weight of the binder, of an oligomer or polymer or both having functional groups capable of reacting with the isocyanate groups of component (a); and
(c) 0-60% by weight, based on the weight of the binder, of an organic polyisocyanate crosslinking agent.
Dispersed gelled acrylic polymers of the foregoing composition are also a part of this invention.
In repairing a clear coat/color coat finish of an automobile or truck, generally the color coat is applied and dried for a short time but not cured and then the clear coat is applied and both coats are cured. If necessary, the cured clear coat is buffed to improve appearance and remove minor imperfections. The coating composition of this invention has a short drying time and thereby improves the rate of processing vehicles through a typical repair facility. In particular, the novel composition has a short tack and dust free time when used as a clear finish so that the vehicle can be moved out of the work area to provide room for another vehicle to be painted. The novel composition when used as a clear finish is buffable in a short period of time after application and initially drying and remains buffable for several days, preferably up to one week before it cures into a hard durable finish. For a finish to be buffable it must be hard but not tough.
Preferably, the coating composition of this invention when used as a clear coat dries to tack free state in about two hours of application and can be buffed in about three hours of application.
The novel coating composition is solvent based and contains about 10-60% by weight of an organic liquid carrier and correspondingly, about 90-40% by weight of film forming binder and preferably has a VOC of about 3.5-4.5 pounds of solvent per gallon of coating composition. The binder contains (a) about 10-70% by weight, preferably 20-60%, most preferably 30-50%, of a dispersed gelled acrylic polymer having isocyanate functionality, (b) about 30-90% by weight, preferably 40-80%, most preferably 50-70%, of an oligomer or polymer or a combination thereof having functional components that are reactive with the isocyanate groups on the dispersed gelled acrylic polymer, and (c) about 0-60% by weight, preferably 0-50%, most preferably 0-30%, of a polyisocyanate crosslinking agent capable of reacting with the functional components on the oligomer or polymer. Herein, binder components (a) plus (b) plus (c) are considered to equal 100 weight percent, and other components are calculated as parts (weight) relative to 100 parts of (a) plus (b) plus (c).
Generally, the novel coating composition is used as a clear coat but can be pigmented with conventional pigments and used as a monocoat or as basecoat.
The dispersed gelled acrylic polymer (also referred to herein as a non-aqueous dispersion or NAD polymer) used to formulate the coating composition of this invention is prepared from a macromonomer which forms the linear stabilizer polymeric components that are chemically grafted to a core.
Preferably, the polymer contains about 30-70% by weight of the core and 70-30% by weight of substantially linear stabilizer polymeric components. These linear stabilizer components are soluble in the organic carrier liquid used to form the coating composition and keep the acrylic polymer dispersed in the liquid while the core is insoluble in this liquid. These macromonomers which form the stabilizer polymeric components of the polymer comprise polymerized alpha-beta ethylenically unsaturated monomers and have one ethylenically unsaturated moiety preferably but not necessarily at the terminal end and have a weight average molecular weight (Mw) of 500-20,000, preferably 1,000 to 10,000. The core, conversely, is formed from a high molecular weight polymer having a weight average molecular weight (Mw) of 50,000 to 500,000, preferably 50,000 to 200,000. About 25-75% (by weight), preferably 40-60%, of the macromonomer is copolymerized with 75-25%, preferably 60-40%, of a blend of other alpha-beta ethylenically unsaturated monomers which form the core of the acrylic polymer.
In the present invention, the core, the stabilizer component, or both contain isocyanate groups that are capable of reacting with the other binder components present in the coating composition. More particularly, at least 3%, preferably 3-30% by weight, of the polymerized monomers in the core, stabilizer component or in both have isocyanate groups attached thereto. The isocyanate groups can be attached by post reaction of isocyanate reactive functional groups in the core, the macromonomer, or both with polyisocyanates, e.g., di- and triisocyanates. The isocyanate groups can also be attached to the dispersed acrylic gelled polymer by copolymerization of isocyanate functional monomers with the core, macromonomer, or both.
In the present composition, while both the stabilizer components and the core may contain isocyanate groups, it is generally preferred to have such reactive functionality only or essentially only or substantially only on the stabilizer components. It is to be understood that the core or macromonomers referred to as having isocyanate functionality may be part of a mixture of core polymers or macromonomers of which a portion do not have any functionality or variable amounts of functionality. It is also understood that, in preparing any core or macromonomers, there is a normal distribution of functionality.
The dispersed gelled acrylic polymer may be, and preferably is, prepared by polymerizing ethylenically unsaturated monomers that comprise the insoluble core in the presence of macromonomers, each macromonomer having at least one ethylenic unsaturation component preferably but not necessary in the terminal component. The acrylic polymer can be envisioned as being composed of a core having a plurality of macromonomer stabilizer components attached thereto.
Macromonomers can be prepared by conventional techniques as shown in Barsotti et al U.S. Pat. No. 5,763,528 issued Jun. 9, 1998 (see Example 2) using conventional catalysts.
In a preferred method for preparing macromonomers, a catalytic chain transfer agent is used to ensure that the resulting macromonomer only has one terminal ethylenically unsaturated group which will polymerize with the core monomers to form the acrylic polymer. Typically, in the first step of the process for preparing the macromonomer, the monomers are blended with an organic solvent and a cobalt chain transfer agent and heated usually to the reflux temperature of the reaction mixture. In subsequent steps additional monomers and conventional polymerization catalyst and optional additional cobalt chain transfer agent are added and polymerization is continued until a macromonomer is formed of the desired molecular weight. The cobalt approach is also described in Barsotti et al U.S. Pat. No. 5,763,528 issued Jun. 9, 1998 (see Example 1)
Preferred cobalt chain transfer agents or catalysts are described in Janowicz et al U.S. Pat. No. 4,680,352 issued Jul. 14, 1987 and Janowicz U.S. Pat. No. 4,722,984 issued Feb. 2, 1988. Most preferred are pentacyanocobaltate (II), diaquabis(borondifluorodimethyl-glyoximato) cobaltate(II) and diaquabis(borondifluorodiphenylglyoximato) cobaltate (II). Cobalt (III) versions of these catalysts are also preferred. Typically these chain transfer agents are used at concentrations of about 5-1000 ppm based on the monomers used.
The macromonomer is preferably formed in a solvent or solvent blend using a free radical initiator and a Co (II) or (III) chelate chain transfer agent. Examples of such solvents are aromatics, ketones, glycol ethers, acetates, alcohols as, e.g., methyl ethyl ketone, isopropyl alcohol, n-butyl glycol ether, n-butyl diethylene glycol ether, propylene glycol methyl ether acetate, propylene glycol methyl ether, and N-butanol.
Free radical initiators such as peroxy- and azo-initiators (0.5-5% weight on monomer) are typically used in the synthesis of the macromonomers in the presence of 2-5,000 ppm (on total monomer) or Co (II) chelate in the temperature range between 70-160xc2x0 C., more preferably azo-type initiators as, e.g., 2,2xe2x80x2-azobis (2,4 dimethylpentane nitrile), 2,2xe2x80x2-azobis (2-methylpropane nitrile), 2,2xe2x80x2-azobis (2-methylbutane nitrile), 1,1xe2x80x2-azo (cyclohexane carbonitrile) and 4,4xe2x80x2-azobis (4-cyanopentanoic) acid.
After the macromonomer is formed as described above, solvent is optionally stripped off and the monomers that comprise the core polymers are added to the macromonomer along with additional solvent and polymerization catalyst. Any of the aforementioned azo-type catalysts can be used as can other suitable catalysts such as peroxides and hydroperoxides. Typical of such catalysts are di-tertiarybutyl peroxide, di-cumylperoxide, tertiaryamyl peroxide, cumenehydroperoxide, di(n-propyl) peroxydicarbonate, peresters such as amyl peroxyacetate and the like. Commercially available peroxy type initiators include, e.g., t-butylperoxide or Triganox(copyright) B from AKZO, t-butylperacetate or Triganox(copyright) FC50 from AKZO, t-butylperbenzoate or Triganox(copyright) C from AKZO, and t-butylperpivalate or Triganox(copyright) 25 C-75 from AKZO.
Polymerization is continued at or below the reflux temperature of the reaction mixture until the acrylic polymer is formed of the desired molecular weight. During the polymerization or afterward, non-solvent(s) for the core are added to form low viscosity sprayable polymer dispersion rather than a polymer solution having a relatively high viscosity which would require further dilution with solvents for spraying thereby increasing the VOC content of the composition. It is generally preferred to have the non-solvent(s) for the core present during the polymerization.
Typical solvents that are non-solvents for the core are aliphatics such as heptane, octane, N-decane, or mineral spirits and the like.
Typical monomers that can be used to form the core or the macromonomers are for example (but not limited to), acrylic and methacrylic acid esters of straight-chain or branched monoalcohols of 1 to 20 carbon atoms. Preferred esters are alkyl acrylates and methacrylates having 1-12 carbons in the alkyl group such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate, 2-ethyl hexyl acrylate, nonyl acrylate, lauryl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, pentyl methacrylate, hexyl methacrylate, 2-ethyl hexyl methacrylate, nonyl methacrylate, lauryl methacrylate and the like. Cycloaliphatic acrylates and methacrylates can be used such as trimethylcyclohexyl acrylate, t-butyl cyclohexyl acrylate, isobornyl acrylate, cyclohexyl methacrylate, isobornyl methacrylate, and the like. Aryl acrylates and methacrylates such as benzyl acrylate and benzyl methacrylate also can be used.
Suitable other ethylenically unsaturated comonomers that can be used for forming the core or macromonomer include: acrylamide and methacrylamide and derivatives as alkoxy methyl (meth) acrylamide monomers, such as methacrylamide, N-isobutoxymethyl methacrylamide, and N-methylol methacrylamide; maleic, itaconic and fumaric anhydride and its half and diesters; vinyl aromatics such as styrene, alpha methyl styrene and vinyl toluene; and polyethylene glycol monoacrylates and monomethacrylates.
Other monomers such as itaconic or maleic anhydride, the half ester thereof, acrylonitrile, allyl methacrylate, aceto acetoxyethyl methacrylate, trialkoxy silyl ethyl methacrylate, reaction products of mono epoxy esters or monoepoxy ethers with alpha-beta unsaturated acids and reaction products of glycidyl (meth) acrylate with mono functional acids up to 22 carbon atoms can be used.
Ethylenically unsaturated epoxy functional monomers can also be used such as glycidyl acrylate and glycidyl methacrylate. Polymerizable acid functional monomers can be used such as acrylic acid, methacrylic acid, maleic acid, itaconic acid and the like. Methacrylic and acrylic acid are preferred. Other acids that can be used are ethylenically unsaturated sulfonic, sulfinic, phosphoric or phosphonic acid and esters thereof; typically, styrene sulfonic acid, acrylamido methyl propane sulfonic acid, vinyl phosphonic or phosphoric acid and its esters and the like, also can be used.
Other functional monomers that can be used for forming the core or macromonomer include ethylenically unsaturated hydroxy functional monomers. Examples of ethylenically unsaturated monomers containing hydroxy groups include hydroxy alkyl acrylates and hydroxy alkyl methacrylates, wherein the alkyl group has 1 to 4 carbon atoms can be used. Suitable monomers include hydroxy ethyl acrylate, hydroxy propyl acrylate, hydroxy isopropyl acrylate, hydroxy butyl acrylate, hydroxy ethyl methacrylate, hydroxy propyl methacrylate, hydroxy isopropyl methacrylate, hydroxy butyl methacrylate, and the like, and mixtures thereof. Hydroxy functionality can also be obtained from monomer precursors, for example, the epoxy group of a glycidyl methacrylate or glycidyl acrylate unit in a polymer. Such an epoxy group may be converted, in a post polymerization reaction with water or a small amount of acid, to a hydroxy group.
Polymerizable amine functional monomers can also be used. Examples of secondary amine functional monomers that can be used include alkylamino alkyl acrylates and methacrylates having 1-8 carbon atoms in the alkyl groups. Suitable monomers include t-butyl amino ethyl acrylate or methacrylate. Polymerizable tertiary amine functional monomers can also be used such as dimethyl amino ethyl methacrylate or acrylate.
In the synthesis of the acrylic polymer small amounts of difunctional alpha-beta unsaturated compounds can also be used as, e.g., allyl methacrylate or acrylate, ethylene glycol dimethacrylate or hexane diol diacrylate.
The core of the acrylic polymer is gelled or crosslinked during its polymerization through the use of any of the aforementioned difunctional monomers, especially allyl methacrylate. Optionally, the gelled polymers can be generated by post reacting polymers having glycidyl epoxy groups in the core with acid functional monomers (or vice versa) or by addition of polyamine such as ethylene diamine, or by post reacting polymers having hydroxy groups in the core with oligomeric di- or triisocyanates such as hexamethylene diisocyanate.
As indicated above, a couple of approaches can be used to introduce the isocyanate groups into the macromonomer or the core or both. Isocyanate groups can be introduced by post reacting isocyanate reactive functional groups in the acrylic polymer (core and/or macromonomer) with polyisocyanate compounds. Examples of isocyanate reactive groups in the polymer are hydroxy and secondary amine groups. Such reactive groups can be built into the core, macromonomer, or both during its polymerization through use of suitable hydroxy or secondary amine functional ethylenically unsaturated comonomers. Any of the aforementioned hydroxy or secondary amine functional monomers can be used to form these isocyanate reactive groups on the acrylic polymer.
When post reacting the polyisocyanate with such isocyanate reactive groups, the reaction conditions should be chosen so that 100% of the forgoing isocyanate reactive functional groups are reacted with the polyisocyanate, or as close to 100% as can be reasonably achieved. It is generally preferred to use excess isocyanate to drive the reaction to completion. This will result in some of the isocyanate molecules being unattached to the dispersed gelled acrylic polymer. Component (a) is this instance will then be a mixture of unreacted isocyanate and isocyanate functional NAD. The equivalent ratio of NCO to OH/NH groups used during synthesis preferably ranges from 5:1 to 50:1. Typically if the ratio is less than 5:1, the stability of the NAD is compromised. If the ratio is greater than 50:1, the amount of NAD particles introduced in the final coating is insufficient to improve the tack free drying time of the coating.
Any conventional aromatic, aliphatic, cycloaliphatic polyfunctional isocyanates having at least two isocyanate groups per molecule, including difunctional isocyanates, trifunctional isocyanates and isocyanate functional adducts of a polyol and a diisocyanate can be used to modify the foregoing isocyanate reactive functionalities and introduce the isocyanate groups in the polymer.
Typically useful diisocyanates are 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, bis cyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 2,3-dimethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopenthylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,3-phenylene diisocyanante, 1,5-naphthalene diisocyanate, bis-(4-isocyanatocyclohexyl)-methane, diisocyanatodiphenyl ether and the like.
Typical trifunctional isocyanates that can be used are triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate, 2,4,6-toluene triisocyanate and the like. Trimers of diisocyanates also can be used such as the trimer of hexamethylene diisocyanate which is sold under the tradename Desmodur(copyright) N-3390, the trimer of isophorone diisocyanate which is sold under the tradename Desmodur(copyright) Z-4470 and the like.
Isocyanate functional adducts can be used that are formed from an organic polyisocyanate and a polyol. Any of the aforementioned polyisocyanates can be used with a polyol to form an adduct. Polyols such as trimethylol alkanes like trimethylol propane or ethane can be used. One useful adduct is the reaction product of tetramethylxylidene diisocyanate and trimtheylol propane and is sold under the tradename of Cythane(copyright) 3160.
In alternate approach, isocyanate groups can be introduced in the acrylic polymer (core and/or macromonomer) by adding ethylenically unsaturated isocyanate functional monomers during polymerization of the macromonomer, the core, or both. Examples of isocyanate functional monomers that can be used to introduce isocyanate groups into the acrylic polymer during its polymerization include isocyanatoethyl methacrylate, isocyanatoethyl acrylate, meta-tetramethyl xylylene isocyanate and the like. While practicing this approach, functional monomers that are reactive with isocyanates must be absent in the core and macromonomer. These functional monomers include any of the aforementioned monomers having hydroxy, amine, or acid groups.
Other possibilities for introducing isocyanate groups into the acrylic polymer (core and/or macromonomer) will be apparent to persons skilled in the art.
In the present invention, the preferred average particle size of the core is in the range of 0.1 to 1 microns, preferably in the range from 0.2 to 0.5 microns.
The core of the acrylic polymer is a gelled structure. Particularly useful acrylic polymers include the following:
an acrylic polymer having a core of polymerized monomers of styrene, methyl methacrylate, glycidyl methacrylate, methacrylic acid, hydroxy ethyl acrylate, methyl acrylate and allyl methacrylate and stabilizing polymeric components of a macromonomer of 2-ethyl hexyl methacrylate, isobornyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, t-butyl aminoethyl methacrylate, and having the gelled polymer post reacted with di- or tri-isocyanates to attach the isocyanate groups thereto.
an acrylic polymer having a core of polymerized monomers as above and stabilizing polymeric components of a macromonomer of butyl acrylate, butyl methacrylate, hydroxy ethyl acrylate, styrene, glycidyl methacrylate, and methacrylic acid, and having the gelled polymer post reacted with a di- or tri-isocyanate to attach isocyanate groups thereto.
an acrylic polymer having a core of polymerized monomers of styrene, methyl methacrylate, methyl acrylate, isocyanato ethyl methacrylate, allyl methacrylate, and glycidyl methacrylate, and stabilizing polymeric components of a macromonomer of styrene, butyl acrylate, butyl methacrylate, isobornyl methacrylate, isocyanato ethyl methacrylate and hydroxy ethyl acrylate.
The coating composition of this invention formed with the above described acrylic polymer dispersion also contains an oligomer or polymer or another dispersed gelled polymer or combination thereof having functional components that are reactive with the isocyanate groups on the dispersed gelled acrylic polymer.
Useful oligomers have a weight average molecular weight of about 200-2,000 and a polydispersity of less than 1.7 and have functional components capable of reacting with the isocyanate groups on the dispersed gelled acrylic polymer.
Typically useful oligomers include hydroxy functional caprolactone oligomers which may be made by reacting caprolactone with a cyclic polyol. Particularly useful caprolactone oligomers are described on col. 4., line 3xe2x80x94col. 5, line 2 of Lamb et al U.S. Pat. No. 5,286,782 issued Feb. 15, 1994. Other useful oligomers are polyester oligomers such as an oligomer of an alkylene glycol, like propylene glycol, an alkane diol, like hexane diol, and an anhydride like methyl hexahydrophthalic anhydride reacted to a low acid number. Another useful oligomer is an acid functional oligomer such as an oligomer of a polyol such as pentaerythritol reacted with an anhydride such as methyl hexahydrophthalic anhydride to an acid number of about 30-300, preferably 150-250. Other useful oligomers are hydroxy functional and are formed by reacting 1,2 epoxy butane with the above described acid functional oligomers using triethyl amine as a reaction catalyst resulting in very low (less than 20) acid number oligomers. Particularly useful hydroxy functional oligomers are described in Barsotti et al U.S. Pat. No. 6,221,494 issued Apr. 24, 2001.
Additional reactive oligomers include aldimine oligomers which are the reaction products of alkyl aldehydes, such as, isobutyraldehyde with diamines, such as isophorone diamine. Ketimine oligomers which are the reaction product of alkyl ketones, such as, methyl isobutyl ketone with diamines, such as, 2-methyl pentamethylene diamine. Polyaspartic esters, which are the reaction product of diamines, such as, isopherone diamine with dialkyl maleates, such as, diethyl maleate. All of the foregoing additional molecules are well known in the art.
Acrylic polymers or polyesters having functional components capable of reacting with isocyanate groups can also be used. It is generally preferred to use such polymers in combination with any of the aforementioned oligomers for improved film integrity. Typically useful acrylic polymers include hydroxy functional acrylic polymers having a weight average molecular weight in the range from 2,000 to 50,000, preferably 3,000 to 20,000 and a Tg preferably in the range of 0xc2x0 C. to 80xc2x0 C., which are made from typical monomers such as acrylates, methacrylates, styrene and the like and functional monomers such as hydroxy ethyl acrylate, glycidyl methacrylate, or gamma methacryly propyl trimethoxy silane and the like.
Typically useful polyesters have a weight average molecular weight in the range from 2,000 to 50,000, preferably from 2,000 to 5000 and a Tg preferably in the range from xe2x88x9220xc2x0 C. to 100xc2x0 C. The polyesters suitable for use in the invention are conventionally polymerized from suitable polyacids, including cycloaliphatic polycarboxylic acids, and suitable polyols, which include polyhydric alcohols. The details of polyesters suitable for use in this invention are provided in Hoffmann et al U.S. Pat. No. 5,326,820 issued Jul. 5, 1994, which is incorporated herein by reference. One of the commercially available polyester, which is particularly preferred, is SCD(copyright) -1040 polyester, which is supplied by Etna Product Inc., Chagrin Falls, Ohio.
Hydroxy functional dispersed gelled acrylic polymers can also be used in the coating composition. Examples of such polymers include acrylic polymers which have a core formed from polymerized monomers of methyl methacrylate, glycidyl methacrylate, methacrylic acid, methyl acrylate and stabilizing polymeric components formed from a macromonomer of styrene, butyl methacrylate, butyl acrylate, hydroxy ethyl acrylate, methacrylic acid, isobornyl methacrylate, and glycidyl methacrylate. The core is formed from a high molecular weight polymer having a weight average molecular weight of 50,000 to 500,000, preferably in the range of from 50,000 to 200,000. The arms make up about 10 to 90 percent of the polymer and are formed from low molecular weight macromonomer having an average molecular weight of in the range from about 500 to 20,000, preferably 3,000 to 20,000. The details of these hydroxy functional dispersed polymers which can be used in the present composition are provided in Barsotti et al. U.S. Pat. No. 5,763,528 (see Examples 1 and 2), which is incorporated by reference herein.
Compatible mixtures of any of the aforementioned oligomers or polymers can also be used.
Optionally, the isocyanate functional dispersed gelled acrylic polymer described above can be combined with an organic polyisocyanate crosslinking agent to enhance the film forming ability of the coating composition. As with the dispersed gelled acrylic polymer, these compounds are reactive with the oligomer or polymer described above. Any of the conventional aromatic, aliphatic, cycloaliphatic diisocyanates, triisocyanates and isocyanate functional adducts of a polyol and a diisocyanate as described above can be used. Blocked polyisocyanates also can be used. Typical blocking agents are alcohols, ketimines, oximes and the like.
In the coating composition of the present invention, the aforementioned isocyanate components, also referred to herein as the activator, are typically stored separately from the other binder components prior to application.
To improve weatherability of the clear composition about 0.1-10% by weight, based on the weight of the binder, of ultraviolet light stabilizers screeners, quenchers and antioxidants can be added. Typical ultraviolet light screeners and stabilizers include the following:
Benzophenones such as hydroxy dodecycloxy benzophenone, 2,4-dihydroxy benzophenone, hydroxy benzophenones containing sulfonic acid groups and the like.
Benzoates such as dibenzoate of diphenylol propane, tertiary butyl benzoate of diphenylol propane and the like.
Triazines such as 3,5-dialkyl-4-hydroxyphenyl derivatives of triazine, sulfur containing derivatives of dialkyl-4-hydroxy phenyl triazine, hydroxy phenyl-1,3,5-triazine and the like.
Triazoles such as 2-phenyl-4-(2,2xe2x80x2-dihydroxy benzoyl)-triazole, substituted benzotriazoles such as hydroxy-phenyltriazole and the like.
Hindered amines such as bis(1,2,2,6,6-pentamethyl-4-piperidinyl sebacate), di[4(2,2,6,6-tetramethyl piperidinyl)] sebacate and the like and any mixtures of any of the above.
The coating composition contains sufficient amount of a catalyst or catalyst blend to cure the composition at ambient temperatures. Generally, about 0.01-2% by weight, based on the weight of the binder, of catalyst is used. Typically useful catalysts are triethylene diamine and alkyl tin laurates such as dibutyl tin dilaurate, dibutyl tin diacetate, tertiary amines and the like.
Generally, flow control agents are used in the composition in amounts of about 0.1-5% by weight, based on the weight of the binder, such as polyacrylic acid, polyalkylacrylates, polyether modified dimethyl polysiloxane copolymer and polyester modified polydimethyl siloxane.
When used as a clear coating, it may be desirable to use pigments in the coating composition which have the same refractive index as the dried coating. Typically, useful pigments have a particle size of about 0.015-50 microns and are used in a pigment to binder weight ratio of about 1:100 to 10:100 and are inorganic siliceous pigments such as silica pigment having a refractive index of about 1.4-1.6.
In the application of the coating composition as a clear coating to a vehicle such as an automobile or a truck, the basecoat which may be either a solvent based composition or a waterborne composition is first applied and then dried to at least remove solvent or water before the clear coating is applied usually by conventional spraying. Electrostatic spraying may also be used. The dry film thickness of the clear coating is about 0.5-5 mils. The clear coating is dried at ambient temperatures generally in less than 5 minutes to a tack and dust free state. Moderately higher temperatures up to about 40xc2x0 C. also can be used. As soon as the clear coating is sufficiently cured to be dust free and tack free the vehicle can be moved from the work area to allow for the refinishing of another vehicle.
Generally, within about 3 hours after application, the clear coating is sufficiently cured to allow for buffing and polishing if needed to remove imperfections and improve gloss of the finish. The clear coating continues to cure and after 7-10 days reaches a relatively high level of hardness and toughness that is required for a durable and weatherable automotive finish.
The coating composition of this invention can also be pigmented and used as a base coat in a clear coat/color coat finish or as a monocoat. Typical pigments that are used in such a coating composition are metallic oxides such as titanium dioxide, iron oxides of various colors, zinc oxide, carbon black, filler pigments such as talc, china clay, barytes, carbonates, silicates and a wide variety of organic colored pigments such as quinacridones, copper phthalocyanines, perylenes, azo pigments, indanthrone blues, carbazoles such as carbazole violet, isoindolinones, isoindolones, thioindigo reds, benzimilazolinones, and metallic flake pigments such as aluminum flake, nickel flake and the like.
Coating compositions of this invention have excellent adhesion to a variety of metallic or non-metallic substrates, such as previously painted substrates, cold rolled steel, phosphatized steel, and steel coated with conventional primers by electrodeposition. These coating composition can be used to coat plastic substrates such as polyester reinforced fiberglass, reaction injection-molded urethanes and partially crystalline polyamides.
Coating compositions of this invention can be applied by conventional techniques such as spraying, electrostatic spraying, dipping, brushing, flowcoating and the like. The preferred techniques are spraying and electrostatic spraying. In refinish applications, the composition is dried and cured at ambient temperatures but can be forced dried at elevated temperatures of 40-100xc2x0 C. for about 5-30 minutes. For OEM applications, the composition is typically baked at 100-150xc2x0 C. for about 15-30 minutes to form a coating about 0.1-3.0 mils thick. When the composition is used as a clearcoat, it is applied over the color coat which may be dried to a tack-free state and cured or preferably flash dried for a short period before the clearcoat is applied. The color coat/clearcoat finish is then baked as mentioned above to provide a dried and cured finish. The present invention is also applicable to non-baking refinish systems, as will be readily appreciated by those skilled in the art.
It is customary to apply a clear topcoat over a basecoat by means of a xe2x80x9cwet-on-wetxe2x80x9d application, i.e., the topcoat is applied to the basecoat without curing or completely drying the basecoat. The coated substrate is then heated for a predetermined time period to allow simultaneous curing of the base and clear coats.
The invention will be further described by reference to the following Examples. All parts and percentages are on a weight basis unless otherwise indicated. All molecular weights disclosed herein are determined by GPC (gel permeation chromatography) using a polystyrene standard.